This invention relates to wireless communications devices and, more particularly, to an improved small, low cost antenna package for such a device.
The greater capacity and larger number of providers for Personal Communications Services (PCS) means far greater competition for wireless subscribers. Although total revenue is soaring, revenue per subscriber has been declining as many casual and emergency-only users enter the market. In response, equipment providers are under pressure to keep terminal costs low, and at the same time support an increasing number of features that will increase revenue per subscriber. Wireless data transmission is one of the growth areas for wireless services, with increasing demand for wireless images, financial information and Internet access. Although a conventional cellular phone can be used as a wireless modem to transmit data, transmission rates are low and bit error rates are high. Subscriber acceptance of data via this mode has been relatively weak. Although the higher frequency and bandwidth of PCS provides some improvement, it does not offer the significant increase in bit rate that makes data transmission attractive to a wide customer base.
Antenna diversity does provide this significant improvement. Spatial diversity with a switching algorithm can increase the system gain by 3-5 dB depending on the effectiveness of the algorithm and the isolation between antennas. As an example, a simple switch algorithm monitors only the one antenna signal in use. When this signal falls below some threshold value, it switches to the other antenna. A more complicated algorithm would monitor both antenna signals and switch to the one with the strongest signal even if they are both above the operational threshold. Even more complicated systems would replicate much of the RF train and monitor both signals closer to digital baseband. The higher average gain attained with switched diversity allows lower bit error rates to be achieved at higher data rates.
Realizing enough separation between the antennas is an important consideration in spatial diversity on a handset. Horizontal separation is more effective than vertical separation because the decorrelation of the received signal increases faster with horizontal separation, particularly when the vertical beamwidth is smaller than the horizontal beamwidth as it is when one of the antennas is an omni-directional dipole. The signals have to be essentially uncorrelated and the first null in correlation factor occurs when the distance between antennas is approximately 0.38 times the wavelength. Practically, a correlation coefficient below 0.25, and in some cases below 0.50, can be neglected, providing effective separations of as little as 1/5 the wavelength. This is about 8 cm at 900 MHz and 4 cm at 1.9 GHz. The problem with diversity in a small terminal with a size less than one half the wavelength is that it is difficult to determine the center of the radiation since the entire housing radiates through near field coupling, especially when the antenna is inside. So although the distances required for effective diversity can be realized on the handset, the actual situation is much more complicated. When the antennas are different types and positioned differently, then other types of diversity (directional and polarization) may have an effect as well.
It is therefore apparent that a need exists for small, low cost antennas for use as diversity antennas in handheld wireless communications devices.